[http://hurd.gnu.org GNU Hurd] reimplements UNIX in a multiserver environment on top of the Mach microkernel. As of version 0.7, the Hurd runs on single-processor ia32 systems. Porting effort to amd64 is underway. Some of the Hurd's components, such as parts of the `ext2fs` file server and the `pfinet` networking server are in fact scavenged from ancient versions of Linux, while some other components are original. The networking stack has a single-process architecture. NIC and disk drivers are also taken from Linux, but run as part of Mach in kernel mode. There are experimental versions of the Hurd with support for running Linux 2.6.x networking and disk drivers in user mode via the DDE compatibility layer. The Hurd's hardware support is rather limited. As of version 0.7, it did not support USB or sound. There are, however, experimental PoC's implementing USB and sound via rump kernels. The Hurd is a multiuser system and a huge emphasis is put on structuring it in such a way that unprivileged users can work with the computer without needing to acquire the administrator's privileges for certain tasks (e.g. making untrusted/experimental file systems accessible in the file system namespace). The Hurd offers great flexibility in the dynamic configuration of the system through the translator mechanism. Users can modify behavior for a file system path by interposing their own functionality (i.e. setting a translator) that gets activated on open. There are two GNU Hurd distributions: Debian GNU/Hurd and Arch Hurd. The former includes about 80% of the Debian packages, which is still in the very much respectable order of tens of thousands of packages.

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HelenOS, on the other hand, is a multiserver environment which does not reimplement any legacy system. It has its own microkernel called SPARTAN. Besides ia32, it also supports amd64, arm32, ia64, mips32, ppc32, sparc32 and sparc64 to varying degrees, some of them in multiprocessor configurations. An overwhelming majority of HelenOS components such as file systems, networking stack, device drivers and GUI have been written specifically for HelenOS, so there is virtually no bloat introduced by various compatibility layers and glue code, neither there is any additional maintenance burden associated with maintaining aging third-party components. The networking stack is decomposed into several processes, each implementing part of the stack such as IP, TCP or UDP. Essentially all HelenOS drivers run in user mode, the exception being kernel drivers used for debugging purposes and the timer and interrupt controller drivers. HelenOS supports both USB and sound (Sound Blaster 16 and Intel HDA). HelenOS is a single user operating system, even though there has been some experimental work done on multiuser support. HelenOS does not have a direct analogy to the translator mechanism, but the system configuration can be dynamically changed by spawning/killing user space servers that implement certain IPC protocols and by stacking services on top of each other. HelenOS currently does not support very many user applications as the development focus is on subsystems, frameworks and drivers. A very limited support exists for running a few standard development tools inside HelenOS (e.g. binutils, gcc, python and some others). See also the following question.

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HelenOS, on the other hand, is a multiserver environment which does not reimplement any legacy system. It has its own microkernel called SPARTAN. Besides ia32, it also supports amd64, arm32, ia64, mips32, ppc32 and sparc64 to varying degrees, some of them in multiprocessor configurations. An overwhelming majority of HelenOS components such as file systems, networking stack, device drivers and GUI have been written specifically for HelenOS, so there is virtually no bloat introduced by various compatibility layers and glue code, neither there is any additional maintenance burden associated with maintaining aging third-party components. The networking stack is decomposed into several processes, each implementing part of the stack such as IP, TCP or UDP. Essentially all HelenOS drivers run in user mode, the exception being kernel drivers used for debugging purposes and the timer and interrupt controller drivers. HelenOS supports both USB and sound (Sound Blaster 16 and Intel HDA). HelenOS is a single user operating system, even though there has been some experimental work done on multiuser support. The closest analogy to the translator mechanism is exposing services in the location service filesystem. The system configuration can be dynamically changed by spawning/killing user space servers that implement certain IPC protocols and by stacking services on top of each other. HelenOS currently does not support very many user applications as the development focus is on subsystems, frameworks and drivers. A very limited support exists for running a few standard development tools inside HelenOS (e.g. binutils, gcc, python and some others). See also the following question.

[http://www.minix3.org MINIX 3] is a multiserver system which comes with its own microkernel and core services, and portions of NetBSD userland. The microkernel does not support threads. As of version 3.3.0, MINIX runs on single-processor ia32 and arm32 systems. Some multiprocessor code is present, but has been unmaintained since version 3.1.x. MINIX 3 comes with two alternative networking stacks. Its own `inet` server and the `lwip` server based on the lwIP networking stack. Even though lwIP contains some experimental support for IPv6, both servers are IPv4 only. IPv6 support for the `lwip` server is work in progress. Both `inet` and `lwip` have monolithic architecture that combines IP, UDP and TCP in one process. Some research has been done on making the networking stack modular. The file systems and most of the device drivers have been developed specifically for MINIX. As of version 3.3.0, MINIX supports USB hubs, USB mass storage and the MUSB OTG controller (as found on some !SoCs). The standard USB host controllers or the USB human interface devices are not supported. There are development versions of MINIX 3 that use Linux 2.6.29 USB drivers running inside the DDE compatibility layer. The selling point of MINIX 3 has been its reliability features. MINIX 3 contains a resurrection service that can, in some situations, restart a crashing or misbehaving service. The development version of MINIX 3 supports live update of the running system services. Thanks to its NetBSD userland, MINIX 3 boasts thousands of packages.

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To avoid much repetition, see also the answer to the previous question. On top of that, the HelenOS kernel implements kernel threads. HelenOS modular networking stack supports both IPv4 and IPv6. The HelenOS USB stack supports UHCI and OHCI. EHCI is supported in a development branch. Drivers for USB hubs, human interface devices (a.k.a. mice and keyboards) and mass storage are available. HelenOS does not come with a resurrection service nor live update.

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To avoid much repetition, see also the answer to the previous question. On top of that, the HelenOS kernel implements kernel threads. HelenOS modular networking stack supports both IPv4 and IPv6. The HelenOS USB stack supports UHCI, OHCI and EHCI. Drivers for USB hubs, human interface devices (a.k.a. mice and keyboards) and mass storage are available. HelenOS does not come with a resurrection service nor live update.